1. Field of the Invention
The present invention relates to a vitreous composition containing a vanadium-based particulate additive, to its method of preparation and to its use as self-healing material, in particular for manufacturing seals in devices operating at high temperature, such as fuel cells and steam electrolyzers.
2. Description of Related Art
Glasses and glass-ceramics are rigid materials widely used in industry, especially for producing seals in various devices that have to operate at high temperature, especially between 500 and 900° C. Among such devices, mention for example may be made of fuel cells (in particular solid electrolyte fuel cells or solid oxide fuel cells (SOFCs)) that operate at temperatures of 700 to 900° C. and steam electrolyzers that can be used for the production of hydrogen and also operate at very high temperature. In these two particular cases, the anode and cathode compartments must be separated and gastight because of the presence of hydrogen and oxygen. During the operation and use of these devices, the glasses and glass-ceramics present therein are subjected to thermal cycles that cause cracks to form. The appearance of these cracks therefore reduces the longevity of the devices incorporating such materials.
Two methods for healing cracks formed in glasses and glass-ceramics have already been proposed.
The first type of method, called “intrinsic self-healing”, consists, without the addition of any healing additive, in filling the cracks or in repairing the surface state of a material of the glass, glass-ceramic or metal/glass composite type by a simple heat treatment in order to physically modify the material, usually to soften it. This treatment is carried out by heating the material or the device containing it to a temperature above the melting or softening point of the material. However, the temperatures used for such melting or softening are usually above the temperatures that the devices incorporating these materials can withstand. Thus, Liu et al. (Journal of Power Sources, 2008, 185, 1193-1200) discuss the heat treatment of SOFC fuel cells that contain glass seals based on a mixture of BaO, SiO2Al2O3, CaO and B2O3, at a temperature above the creep temperature of the glass in order to repair the cracks formed, while indicating, however, that one limitation of this method is that the need to raise the temperature in order to make the glass creep causes prejudicial deformations of the system into which it is incorporated.
The second type of method, called “extrinsic self-healing”, consists in adding a healing additive to the composition of the material which enables the cracks to be filled by a chemical reaction of said additive. This second method applies to materials of the polymer and ceramic/composite type. Thus, U.S. Pat. No. 5,965,266 describes the production of a fibrous material reinforced by a ceramic matrix that comprises a self-healing phase containing at least one glass precursor, for example carbon tetraboride (B4C) or an SiBC system, and free carbon (10 to 35%). An oxidizing atmosphere, at a temperature of at least 450° C. but not exceeding 850° C., causes the carbon to oxidize, thereby subsequently transforming the self-healing phase into a glass so as to fill the cracks possibly present in the material. The above document therefore teaches that a composite material containing a glass precursor having the ability to be oxidized at a temperature of 850° C. or below, such as a precursor based on boron and/or silicon, is self-healing in an oxidizing atmosphere when the precursor composition used during its formation contains 15 to 35% carbon. Moreover, White et al. (Nature, 2001, 409, 794-797) describe a self-healing polymer material (polydicyclopentadiene) comprising a microencapsulated polymerizable self-healing agent (dicyclopentadiene) which is released upon appearance of a crack. The presence of a polymerization catalyst (Grubbs catalyst) in the structure of the polymer material is necessary in order to cause the healing agent to polymerize, at room temperature, and to fill the cracks. The material obtained after healing is, however, less resistant than in the initial state. Moreover, this healing technique must be carried out at room temperature and provides healing at any point in the material only if the polymerizable self-healing agent and the catalyst are in immediate proximity of each other and both uniformly distributed in the structure of the material.